Isolation of intracytoplasmic A particles from primary and transplanted mouse mammary tumors. Biochemical, immunologic, ultrastructural, and biologic characterization of the purified particles and comparison of these results with those obtained with similarly purified mouse mammary tumor virus (MMTV). A particle proteins were isolated from SDS-polyacrylamide gels, three MMTV antigenic determinants P27, P14, P10 were demonstrated to be associated with A particle structural polypeptides by immunodiffusion. A precursor-product relationship between A particles and MMTV nucleoprotein cores is becoming apparent. BIBLIOGRAPHIC REFERENCES: Longfellow, D.G. and Smith, Gilbert H.: Further Characterization of Intractyoplasmic A particle Associated DNA. J. Virol. 17: 816-823, 1976. Schneider, W.C. and Smith, G.H.: On the Isolation of Zymogen Granules by Zonal Ultracentrifugation and Their Nucleic Acid Content. Analyt. Biochem. (1977) in press.